This invention relates generally to secondary emission cathodes and more particularly to a semiconductor secondary emission cathode in a high-power cross-field tube which requires a cathode capable of providing high current density.
The prior art secondary emission cathodes made of very thin insulating films, BeO, AlO and MgO for example, with thickness approximating 50 Angstroms, possess enhanced conductivity due to tunneling. Therefore, they are capable of providing high current densities (approximately 1 to 10 amperes per square centimeter) which allows these films to be used as secondary emission cathodes in crossed-field high power tubes. However, these thin films are eroded away by electron bombardment in a relatively short time. These films are typically of a material such as magnesium oxide which have a limited life in their application to high power tubes and require extensive time for out-gassing the tube during manufacture in order to allow them to be used at high powers. In order to increase the longevity of the cathode but without improving the out-gassing problem, thicker films for the cathode are desired. Thicker films introduce problems with respect to the effective conductivity of such films which results in the presence of charging effects within the films and an impairment of the available current density relative to that obtained from the very thin insulating films. One attempt in the prior art to the solution of the problem of obtaining greater electronic conduction in thick insulating films is to introduce metallic particles in the insulating film. An example of such a material is magnesium oxide containing gold particles. The metallic particles do result in improved conductivity of the material. However, there is a significant degradation in the secondary emission ratio. In addition, the slight increase in thickness allowed by the addition of metallic particles would not be expected to meet the requirements for a long-life cathode.
It is therefore an object of this invention to provide a secondary emission cathode which is capable of operating at high current density and has a long life because its enhanced conductivity allows a thicker cathode to be used. It is a further object of this invention to provide a secondary emission cathode which will withstand the electron bombardment experienced in its use in a high-power crossed-field vacuum tube. It is a feature of this invention that the out-gassing time of a tube constructed using the semiconductor cathode is small relative to prior art cathodes since there is no oxygen in the semiconductor cathode in contrast with the thin film oxide cathodes. It is a further feature of this invention that pulsed operation of the tube of this invention has an output pulse with fast rise time and non-discernable jitter of the leading edge of the pulse as measured within the few nanosecond limitations of the instrumentation.